The invention relates to the optical measurement of the concentration of a component of interest in a substance to be analyzed, for example the concentration of oxygen in human blood. Typically, this type of optical measuring apparatus includes a housing provided with a monochromator which furnishes monochromatic excitation radiation, an indicator chamber containing an indicator through which the monochromatic excitation radiation passes in order to excite the indicator, and a light-measuring unit which receives the radiation emitted from the indicator in order to determine its spectral (color or fluorescent) response to the concentration of the component of interest. The indicator space is closed off at the side facing the substance to be analyzed by means of a membrane which is permeable for the component whose concentration is to be ascertained; the indicator space is closed off at the side facing the monochromator by means of a wall which is transmissive for the measuring radiation.
With measuring apparatuses of this type, a difficulty results from the fact that the layer of indicator adjoining the permeable membrane in question comes relatively fast into combination with the component whose concentration is to be ascertained; in contrast, the layers of the indicator more remote from the permeable membrane are reached by the component of interest only as the component of interest diffuses, and relatively slowly, into these more remote layers. Accordingly, the equilibrium state which should be reached in order that the concentration read-out of the device be sufficiently accurate, requires a relatively long time to be established.